In a conventional rear wheel driven vehicle, the engine acts through a transfer case to drive a propeller shaft. The propeller shaft extends back to the rear wheel axles where it engages and turns the axles (through a rear differential) and then the rear wheels. The simple driving of the rear wheels provides all the traction that is required for well over 90% of vehicle travel that takes place. Furthermore, to drive all four wheels requires a second propeller shaft projected forwardly to turn the front wheel axles and in turn the front wheels. Obviously, a front and rear wheel drive set up means more moving parts, more parts to wear out and maintain etc.
Even though rear wheel drive only is satisfactory most of the time, there are driving conditions where a four wheel drive vehicle is necessary e.g. off-road driving, winter driving on snow covered roads, etc. The extra traction that is provided with four wheels versus two wheels makes a significant difference under such conditions. Thus, individuals who drive under these latter road conditions e.g. snow skiers, hunters and the like, require the extra traction of four wheel driving.
To accommodate the market for four wheel drive vehicles, it has to be appreciated that most of the driving of such vehicles will still occur on roads where simple rear wheel driving is not only adequate but preferable. This situation has dictated the design of four wheel drive vehicles that includes a clutch mechanism for engaging and disengaging the vehicle's drive mechanism with the front wheels (or rear wheels if the vehicle is designed with full-time front wheel drive). such clutch mechanism is typically provided at two locations i.e. at the transfer case where the front propeller shaft is engaged and disengaged with the drive mechanism of the engine, and at the wheel hub of each front wheel where the front wheels are engaged and disengaged with the front wheel axles. To disengage one and not the other would result in the continued driving of the front propeller shaft, differential mechanism and front axles, either by the front wheels (with transfer case disengaged only) or by the engine drive mechanism (with front wheel disengagement only).
Typically in a four wheel drive vehicle, a two part clutch engagement and disengagement mechanism is employed. A shift lever is provided in the vehicle's cab that is linked to the transfer case as one part of the clutch mechanism, and a hub lock mechanism is provided in each front wheel hub as the second part of the clutch mechanism. Note, however, that the hub lock mechanism is accessible to the outside of the front wheels only and is engaged manually, requiring the driver to stop the vehicle, get out of the vehicle cab and manually engage the hub lock of each front wheel.